Web sensing and guiding apparatus

ABSTRACT

A web sensing and guiding apparatus in which a web position sensor is provided to sense the lateral position of a web as it exits from a web guide roller in an exiting plane. The sensor is mounted for movement along a crosswise guideway that moves with the exiting roller to maintain the web position sensor at a preselected spacing and attitude in relation to the exiting plane of the web, and the sensor is operatively connected to a stationary structure in a manner to maintain the sensor at a preselected location laterally of the path of web travel while accommodating movement of the sensor angularly and longitudinally relative to the path of web travel.

[ 1 3,724,732 Apr. a, 1973 [54] WEB SENSING AND GUIDING APPARATUSPrimary Examiner-Richard A. Schacher Attorney-Vernon J. Pillote et al.

[75] Inventor: William A. Bonner, Rockford, Ill.

[73] Assignee: Rockford [57] ABSTRACT A web sensing and guidingapparatus in which a web Servo Corporation,

Rockford, Ill.

[22] Filed: Mar. 1, 1972 position sensor is provided to sense thelateral position of a web as it exits from a web guide roller in anexit- [21] ing plane. The sensor is mounted for movement along acrosswise guideway that moves with the exiting Appl. No.: 230,576

52 us. roller to maintain the Web Position Sensor at a preselectedspacing and attitude in relation to the exit- [51] Int. Cl.

ing plane of the web, and the sensor is operatively connected to astationary structure in a manner to [58] Field ofSearch..................

maintain the sensor at a preselected location laterally [56] Referencescued of the path of web travel while accommodating move- UNlTED STATESPATENTS ment of the sensor angularly and longitudinally relative to thepath of web travel.

....226l2l X .226/21 12 Claims, 7 Drawing Figures t Ut R0 2 77 99 11 3266 2 88 33 PATENTEUAPR 3 I975 SHEET 2 [1F 2 WEB SENSING AND GUIDINGAPPARATUS BACKGROUND OF THE INVENTION It is common practice in guidinglongitudinally travelling webs to utilize a web guide roller mounted forcombined crosswise and angular movement relative to the path of webtravel, and to sense the lateral position of the web and automaticallymove the web guide roller in a direction to correct deviation of the webfrom the desired path of web travel. Although many web guide apparatushave the web position sensor located to sense the lateral position ofthe web in advance of the exiting web guide roller, it is desirable tosense the position of the travelling web as it exits from the exitingweb guide roller so that the sensor responds to the corrected path ofweb travel. However, when the web leaves the web guide roller at asubstantial angle to the plane of motion of the web guide roller, theexiting web undergoes a substantial shifting movement in a directionnormal to the plane of the exiting. web, and which movement markedlycomplicates the problem of sensing the position of the exiting web. Someweb position sensors, such as gap type web edge sensors, can accommodateonly limited shifting of the web in a direction normal to its plane, andsome other sensors such as reflectivetype photoelectric web edge andline detectors, can accommodate almost no shifting of the web normal toits plane.

SUMMARY OF THE INVENTION The present invention relates to a web positionsensing and guiding apparatus in which a web guide roller is mounted forcombined crosswise and angular movement relative to the path of webtravel. The lateral position of the web is sensed at a location in theexiting plane of the web and power mechanism is provided responsive tothe web position sensor for moving the web guide roller in a directionto correct deviation of the web from a desired path of travel. Thepresent invention is characterized by the provision of a crosswisesensor guide that parallels the web guide roller and which moves as aunit therewith in combined crosswise and angular movement relative tothe path of web travel, a carriage supporting the web position sensorfor movement along the crosswise guide so that the sensor remains in auniform spatial and angular relationship to the exiting web, and amechanism connecting the sensor carriage to a stationary supportstructure to maintain the sensor at a fixed location laterally of thepath of web travel while accommodating movement of the sensor in adirection longitudinally and angularly relative to the path of webtravel. The carriage for the web position sensor is connected to thestationary support structure by a longitudinal guide that parallels thepath of web travel and a swivel connection that slidably en gages thelongitudinal 'sensor guide and swivelly engages the carriage at alocation adjacent the sensing head.

An important object of this invention is to provide a web positionsensing and guiding apparatus in which the sensor is arranged to sensethe web position in the exiting plane of the web and wherein the sensoris mounted so as to maintain a uniform spatial and angular relationshipbetween the web position sensor and the plane of the. exiting web at afixed location laterally of the path of web travel.

A more particular object of this invention is to provide a web positionsensing and guiding apparatus in which a crosswise sensor guidewayparallels the exiting web guide roller and is movable as a unittherewith in combined crosswise and angular movement relative to thepath of web travel, and wherein the web position sensor is mounted formovement along the sensor guideway so as to maintain a uniform spatialand angular relationship to the plane of the web exiting from the webguide roller, and the sensor is connected 'to a stationary supportstructure in a manner to maintain the web position sensor at a presentposition laterally of the path of web travel while accommodatingmovement of the sensor longitudinally and angularly relative to the pathof web travel.

Yet another object of this invention is to provide a web positionsensing and guiding apparatus which enables locating the web positionsensor at a location tangent to the web guide roller to sense theposition of the web as it exits from the web guide roller.

Still another object of this invention is to provide a web positionsensing and guiding apparatus in accordance with the foregoing objects,and which is adjustable for use with webs of different widths.

These, together with other objects and advantages of this invention willbe more readily understood by reference to the following detaileddescription when taken in connection with the accompanying drawingwherein:

FIG. 1 is a plan view of a web position sensing and guiding apparatusembodying the present invention;

FIG. 2 is a side elevational view of the web position sensing andguiding apparatus of FIG. 1;

FIG. 3 is a fragmentary side elevational view taken on the plane 3-3 ofFIG. 1 and illustrating the web position sensing apparatus on a largerscale;

FIg. 4 is a fragmentary end elevational view taken on the plane 4-4 ofFIG. 1 and illustrating portions of the web position sensing apparatuson a larger scale;

FIG. 5 is a fragmentary horizontal sectional view taken on the plane 5-5of FIG. 2 and illustrating the web position sensing apparatus on alarger scale;

FIG. 6 is a fragmentary vertical sectional view taken on the plane 6-6of FIG. 1 and showing the parts on a larger scale; and

FIG. 7 is a fragmentary bottom view taken on the plane 7- 7 of FIG. 2and illustrating the parts on a larger scale.

The present invention relates to a web position sensing and guidingapparatus for use in a web handling system to laterally guide alongitudinally travelling web W along a predetermined longitudinal pathof travel P paralleling the centerline of the web handling system, asthe web exits from the web guide apparatus. The web guide apparatusincludes at least one web guide roller 21 extending transverse to thelongitudinal path of web travel and mounted for axial rotation as bybearings 31 on a movable guide frame 22. The guide frame 22 is mountedon a stationary support structure 23 to sup port the roller 21 forcombined crosswise and angular movement relative to the path of webtravel in a plane of motion M parallel to the roller axis, and the webguide apparatus is arranged in the web handling system so that the webexits from the roller 21 in an exiting plane Wx transverse to the planeof motion of the web guide roller 21. A web position sensor 24 isprovided for sensing a selected longitudinal characteristic of the web,such as the web edge or a line or other longitudinal characteristic ofthe web, and a control apparatus 25 is connected to the web positionsensor 24 and responds to the signals produced thereby to operate anactuator 26 for moving the movable guide frame 22 relative to thestationary support structure 23 in a direction and amplitude to correctdeviation of the web from the desired longitudinal path of web travel.In order to have the web position sensor automatically respond tocorrections in the path of web travel introduced by the web guideapparatus, it is desirable to sense the lateral position of the web asit exits from the web guide apparatus. However, when the web exits fromthe web guide roller in a direction transverse to the plane of motion ofthe web guide roller, the exiting web is not only shifted laterally inthe plane of the web,

but is also shifted in a direction normal to the plane of the web. Thislast-mentioned shifting motion, in a direction normal to the plane ofthe web as it exits from the web guide apparatus, is herein referred toas passline fluctuation of the web. The pass-line fluctuation varies asa function of the angle through which the web guide roller is tilted andthe width of the web. in many installations, particularly in thoseinvolving wide webs or web guide apparatus which tilt throughsubstantial angles, the pass-line fluctuation becomes quite large. Manyweb position sensors such as gap-type photoelectrio and pneumatic webedge sensors can accommodate only a limited pass-line fluctuation andothers, such as vacuum type sensors and reflective-type photoelectricsensors, can accommodate substantially no pass-line fluctuation. Inaccordance with the present invention, the web position sensor 24 ismounted for angular movement with the web guide roller so as to maintaina preselected spacing and angular relationship between the web positionsensor and the exiting web Wx, and the web position sensor is alsosupported for movement crosswise of the path of web travel and parallelto the web guide roller and is operatively connected to the stationarysupport in a manner to maintain the web position sensor at a preselectedposition laterally of the path of web travel.

Various different arrangements can be utilized to support the web guideroller 21 for combined crosswise and angular movement relative to thepath of web travel. in the preferred embodiment illustrated, the movableguide frame 22 is supported on the stationary support structure 23 sothat, during movement of the movable guide frame, the web guide roller21 is disposed generally tangent to an arcuate path having an effectivepivot center designated C and located in advance of the web guide roller21. The particular mechanism herein shown for supporting the web guideapparatus is generally of the type disclosed in the U. S. Pat. to RobertW. Ott, Jr. No. 3,390,823 and includes a plurality of arcuate guide bars32 mounted on the stationary support structure 23 and followers 33 whichslidably engage the respective guide bars and are connected to themovable support structure. As more fully disclosed in aforementionedpatent, the arcuate guide bars 32 are formed and arranged to have theirarc centers disposed at the pivot center C. The actuator 26 shown hereinis of the fluid or hydraulic type and includes a cylinder connected atone end to the stationary support structure and having a piston rodconnected at the other end to the movable guide frame to move the guideframe crosswise and angularly relative to the stationary supportstructure about the pivot center C. As previously described, the web Wexits from the roller 21 in an exiting plane Wx transverse to the planeof motion of the web guide roller 21 and, as shown in FIG. 2, passesover an idler roller 34 rotatably supported at 34a in a stationarysupport structure 23a. The web guide apparatus illustrated herein alsoincludes an entering web guide roller 21a mounted for rotation injournals 31a on the movable guide frame 22, with the roller 21a disposedgenerally tangent to the pivot center C of the exiting web guide roller21. The web W enters the web guide apparatus and passes over an idlerroller 35 rotatably supported at 35a on the stationary support structure23a at a location such that the entering web span We is disposedsubstantially in the plane of the pivot center C and transverse to theplane of motion of the web guide rollers.

The web position sensor 24 and the control 25 therefore can be of anysuitable construction capable of sensing lateral deviation of a selectedlongitudinal characteristic of the web relative to the head 24a of thesensor and may, for example, be a gap-type pneumatic web edge sensor andcontrol such as disclosed in the U. S. Pat. to Robert W. Ott, Jr. No.3,407,706. In the preferred embodiment, however, the web positionsensing device 24 is a reflective-type photoelectric sensor in which thelight transmitter and the photoelectric receiver are both located in thesensor head 24a. When a gap-type sensor is utilized, the sensor must bepositioned slightly downstream of the exiting web guide roller 21 toenable positioning of the gap-type sensor at opposite sides of the webedge. In the reflective-type sensor, however, the sensor can bepositioned adjacent the periphery of the web guide roller 21 at alocation adjacent the point of tangency of the exiting web with the webguide roller. It is, of course, desirable to position the web positionsensor as closely as possible to the exiting web guide roller, andpreferably at the point of tangency thereto, to minimize the tendency ofthe web guide apparatus to hunt and to also minimize fluttering of theweb in a direction normal to its plane at the sensing location.

A crosswise sensor guide is mounted on the movable guide frame 22 forcrosswise and angular movement therewith and parallels the exiting webguide roller 21. In the embodiment shown, the crosswise guide comprisesa pair of guide rods 41, 41a mounted at opposite ends by brackets 42,42a on the movable guide frame 22. As shown, the guide rods 41, 41a aredisposed in a plane generally paralleling the plane of the exiting webWx and are spaced outwardly from the pivot center C of the exiting webguide roller. A sensor carriage 44 is slidably mounted by bearings 45,preferably of the antifriction type, on the guide rods 41, 41a forsliding movement therealong in a path paralleling the web guide roller21. As best shown in FIGS. 3 and 6, the web position sensor 24 isdisposed intermediate the guide rods 41, 41a and is mounted, as byfasteners 46 that extend through slotted openings in brackets 47 on thecarriage, for adjustment in a direction paralleling the plane of motionof the exiting web guide roller 21 and perpendicular to the exitingplane Wx of the web, so as to enable adjustment of the spacing of thesensing head 24a relative to the exiting web and web guide roller 21.The web position sensor is thus movable along the guide rods 41, 41acarried by the movable guide frame so that the web position sensor ismaintained at fixed spacing and angle to the exiting web, during tiltingmovement of the exiting web guide roller.

Provision is made for connecting the sensor carriage 44 to thestationary support structure in such a manner as to cause the sensorcarriage. to move along the guide rods 41, 410 during. the crosswise andangularmovement of the guide roller 21, in a manner to maintain thesensing head 24a at a preselected position laterally of the path of webtravel, while yet accommodating movement of the sensor head angularlyand longitudinally relative to the path of web travel. For this purpose,the carriage 44 for the web position sensor has a bracket 48 rigidtherewith and which extends to a point adjacent the plane of the exitingweb Wx, but at the side of the exiting web guide roller 21 opposite theside from which the web exits therefrom. A swivel bracket 51 is securedto the carriage bracket 48 to define a sensor pivot axis X (FIGS. 3 and4) which is perpendicular to the plane of motion of the web guide roller21 and aligned with the plane of the exiting web W x at a locationadjacent the sensing head 24a of the web position sensor. The sensorpivot axis is connected to the stationary support structure in a mannerto maintain the pivot axis at a preselected location laterally of thepath of web travel while accommodating movement of the carriage 44 in adirection longitudinally and angularly relative to the path of webtravel. For this purpose, the sensor pivot bracket 51 has a follower 52which is slidably supported on a longitudinal guide rod 53 secured tothe stationary support structure to extend transverse to the plane ofthe exiting web. More specifically, the longitudinal guide rod extendsparallel to the plane of motion of the roller 21 and parallel 'to thelongitudinal path of travel of the web. A swivel connection is providedbetween the follower 52 and the bracket 51 and, in the embodiment shown,the follower is supported in a semispherical bearing 52a of theself-aligning type, which allows limited angular movement of the pivotbracket 51 relative to the follower 52 about the sensor pivot axis X.Thus, as the movable guide frame moves about the pivot center C to causethe exiting web guide roller 21 to move crosswise and angularly withrespect to the path of web travel, the sensor 24 and sensor carriage44are maintained by the crosswise guides 41, 41a at a uniform spatialand angular relationship with respect to the exiting web. However, thepivot axis X of the sensor is maintained at a preset position laterallyof the path of web travel so that the sensor carriage and sensor aremoved along the guides 41, 410 during turning movement of the exitingweb guide roller. The longitudinal guide 53 is advantageously mountedfor adjustment in a direction crosswise of the stationary supportstructure, to accommodate webs in which the longitudinal characteristicbeing sensed has a different spacing than the longitudinal path of webtravel, and, as shown, the guide rod 53 is attached to the bracket 58which is slidably adjustable along a bar 59 secured to the stationarysupport structure by brackets 60 to extend crosswise thereof. The bar 59preferably has a noncircular crosssection so as to nonrotatably butslidably support the bracket 58 thereon and a locking screw 61 isprovided for selectively locking the longitudinal guide bar at apreselected position laterally relative to the path of web travel.

OPERATION From the foregoing it is thought that the construction andoperation of the web guide apparatus will be readily understood. The webposition sensor 24 is positioned to sense a preselected longitudinalcharacteristic of the web such as the web edge and to produce an outputsignal correlative with the direction and amount of deviation of the webposition relative to the head 24a of the web edge sensor. The controlapparatus 25 operates the actuator 26 in accordance with the signalproduced by the web position sensor to move the movable guide frame 22and roller 21 crosswise and angularly relative to the path of web travelabout the pivot center C in a direction and amplitude to correctdeviation of the web from the desired path of travel. For example, ifthe web edge in the exiting plane Wx deviates in one direction such asto the left of the effective center of the head 24a of the web positionsensor as viewed in FIG. 1, the sensor operates through the control 25and actuator 26 to shift the exiting web guide roller crosswise andangularly in the opposite direction to shift the web (to the right asviewed in FIG. 1) to correct deviation of the web from the desired pathof web travel. The sensor carriage 44 will move angularly with themovable guide frame 22 so as to maintain the sensing head 24a in uniformspatial and angular relationship to the exiting web Wx. However, thelongitudinal guide rod 53 engaging thepivot bracket 51 maintains thepivot axis X of the sensor head at a fixed location laterally of thepath of web travel. Consequently, as the exiting web guide roller 21 andguide rods 41 shift in one direction crosswise of the path of webtravel, the longitudinal guide rod 53 and pivot bracket locating of theweb position sensor at a position closely adjacent the exiting web guideroller 21 or, as in the preferred embodiment illustrated, tangent to theexiting web guide roller at the point where the web leaves the web guideroller.

The invention has been described in considerable detail with referenceto the preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a web sensing and guiding apparatus for laterally guiding alongitudinally travelling web along a predetermined path of web travelincluding a web guide roller extending transverse to the travelling webto have the web wrap partway around the roller and exit therefrom in anexiting plane, a movable guide frame mounting the web guide roller on astationary support structure for combined crosswise and angular movementrelative to said path of web travel, a web position sensor having asensor head and operative to sense lateral deviation of a selectedlongitudinal characteristic of the web relative to said sensing head,and power means controlled by said web position sensor for moving theguide frame and web guide roller relative to said stationary support ina direction to correct deviation of said web from said desired path ofweb travel; the improvement comprising: means on the movable guide framedefining a crosswise guideway extending crosswise of the path of webtravel parallel to said web guide roller and movable in unison with themovable guide frame in combined crosswise and angular movement relativeto said path of travel, a sensor carriage mounted on said crosswiseguideway for movement therealong parallel to said web guide roller,means mounting said web position sensor on said sensor carriage withsaid sensing head arranged to sense the lateral position of the web at asensing location in said exiting plane of the web, and sensorpositioning means connected to said stationary support structure andoperatively engaging said sensor carriage at a location adjacent saidsensing location for moving the sensor carriage along said crosswiseguideway as the guide frame is moved relative to the stationary supportstructure to maintain the sensor head at a preselected positionlaterally of said path of web travel.

2. A web sensing and guiding apparatus according to claim 1 wherein saidsensor positioning means includes a longitudinal sensor guideway mountedon said stationary support structure and extending transverse to theplane of the exiting web, and means slidably engaging said longitudinalsensor guideway and swivelly engaging said carriage at a locationadjacent the plane of the exiting web.

3. A web sensing and guiding apparatus according to claim 2 wherein saidlongitudinal sensor guideway is mounted on said stationary supportstructure for adjustment crosswise of said path of web travel.

4. A web sensing and guiding apparatus according to claim 2 wherein saidsensor has its sensor head positioned adjacent the periphery of said webguide roller to sense the position of the web as it exits from theroller.

5. A web sensing and guiding apparatus according to claim 1 wherein saidcrosswise sensor guideway includes spaced parallel guide rails mountedon said guide frame parallel to said web guide roller, said carriagebeing slidably mounted on said spaced guide rails.

6. in a web sensing and guiding apparatus for laterally guiding alongitudinally travelling web along a predetermined path of web travelincluding a web guide roller extending transverse to the travelling webto have the web wrap partway around the roller and exit therefrom in anexiting plane, a movable guide frame mounting the web guide roller on astationary support structure for combined crosswise and angular movementrelative to said path of web travel, a web position sensor having asensor head and operative to sense lateral deviation of a selectedlongitudinal characteristic of the web relative to said sensing head,and power means controlled by said web position sensor for moving theguide frame and web guide roller relative to said stationary support ina direction to correct deviation of said web from said desired path ofweb travel; the improvement comprising: at least one crosswise guide barmounted on said movable guide frame for movement therewith and extendingcrosswise of said path of web travel and parallel to said guide roller,a sensor carriage mounted on said crosswise guide bar for slidingmovement therealong, means mounting said web position sensor on saidsensor carriage with said sensing head arranged to sense the lateralposition of the web at a sensing location in said exiting plane of theweb, a longitudinal sensor guide bar mounted on said stationary supportstructure and extending transverse to the plane of the exiting web, andsensor positioning means slidably engaging said longitudinal sensorguide bar and swivelly engaging said sensor carriage at a locationadjacent said sensing head for moving said sensor carriage along thecrosswise guide bar as the guide frame is moved relative to saidstationary support structure to maintain said sensing head at presetposition laterally of said path of web travel.

7. A web sensing and guiding apparatus according to claim 6 wherein saidweb guide roller is supported for movement in a plane of motiontransverse to the plane of the exiting web, said sensor positioningmeans defines a pivot axis transverse to said plane of motion andsubstantially aligned with the exiting plane of the web at said sensinglocation.

8. A web sensing and guiding apparatus according to claim 7 wherein saidweb position sensor has its sensing head positioned adjacent theperiphery of said web guide roller to sense the lateral position of theweb as it exits from the roller.

9. A web sensing and guiding apparatus according to claim 6 wherein saidweb position sensor has its sensing head positioned adjacent theperiphery of said web guide roller to sense the lateral position of theweb as it exits from the roller.

10. A web sensing and guiding apparatus according to claim 6 including astationary guide bar mounted on said stationaiy support structure andextending crosswise of said path of travel, and means mounting saidlongitudinal guide bar for adjustment along said stationary guide bar.

11. In a web position sensing and guiding apparatus including an exitingweb guide roller extending transverse to a travelling web, a stationarysupport structure, means including a movable frame mounting said exitingweb guide roller for movement in a plane of motion in which the exitingweb guide roller is disposed generally tangent to a curved path about aneffective pivot center, said exiting web guide roller in themid-position thereof extending perpendicular to the desired path of webtravel and being arranged to have the travelling web extend partiallytherearound and exit therefrom in an exiting plane transverse to saidplane of motion, a web position sensor having a sensor head andoperative to sense lateral deviation of a selected longitudinalcharacteristic relative to the sensing head, and power means operated bythe web position sensor for moving said movable frame and said exitingweb guide roller in a direction to correct deviation of the web edgefrom said preselected lateral position, the improvement comprising: atleast one crosswise guide bar mounted on said movable frame for movementtherewith and extending parallel to said exiting web guide roller, asensor carriage mounted on said crosswise guide bar for sliding movementtherealong, means mounting said web position sensor on said sensorcarriage with said sensing head arranged to sense the lateral positionof the web at a sensing location in said exiting plane of the web, alongitudinal sensor guide bar mounted on said stationary support toextend transverse to the plane of the exiting web and parallel to theplane of motion of said exiting web guide roller, a follower slidablyengaging said longitudinal guide bar, and pivot means connecting saidfollower to said sensor carriage for relative pivotal movement about anaxis perpendicular to said plane of motion of the exiting web guideroller and substantially aligned with the plane of the exiting web atsaid sensing location for moving said sensor carriage along the sensorguide bar as the guide frame is moved relative to the stationary supportstructure to maintain the sensing head at a preset position laterally ofthe path of web travel.

12. A web position sensing and gliding apparatus according to claim 11including an entrance web guide roller mounted on said movable frameparallel to said exiting web guide roller and generally tangent to saideffective pivot center.

1. In a web sensing and guiding apparatus for laterally guiding alongitudinally travelling web along a predetermined path of web travelincluding a web guide roller extending transverse to the travelling webto have the web wrap partway around the roller and exit therefrom in anexiting plane, a movable guide frame mounting the web guide roller on astationary support structure for combined crosswise and angular movementrelative to said path of web travel, a web position sensor having asensor head and operative to sense lateral deviation of a selectedlongitudinal characteristic of the web relative to said sensing head,and power means controlled by said web position sensor for moving theguide frame and web guide roller relative to said stationary support ina direction to correct deviation of said web from said desired path ofweb travel; the improvement comprising: means on the movable guide framedefining a crosswise guideway extending crosswise of the path of webtravel parallel to said web guide roller and movable in unison with themovable guide frame in combined crosswise and angular movement relativeto said path of travel, a sensor carriage mounted on said crosswiseguideway for movement therealong parallel to said web guide roller,means mounting said web position sensor on said sensor carriage withsaid sensing head arranged to sense the lateral position of the web at asensing location in said exiting plane of the web, and sensorpositioning means connected to said stationary support structure andoperatively engaging said sensor carriage at a location adjacent saidsensing location for moving the sensor carriage along said crosswiseguideway as the guide frame is moved relative to the stationary supportstructure to maintain the sensor head at a preselected positionlaterally of said path of web travel.
 2. A web sensing and guidingapparatus according to claim 1 wherein said sensor positioning meansincludes a longitudinal sensor guideway mounted on said stationarysupport structure and extending transverse to the plane of the exitingweb, and means slidably engaging said longitudinal sensor guideway andswivelly engaging said carriage at a location adjacent the plane of theexiting web.
 3. A web sensing and guiding apparatus according to claim 2wherein said longitudinal sensor guideway is mounted on said stationarysupport structure for adjustment crosswise of said path of web travel.4. A web sensing and guiding apparatus according to claim 2 wherein saidsensor has its sensor head positioned adjacent the perIphery of said webguide roller to sense the position of the web as it exits from theroller.
 5. A web sensing and guiding apparatus according to claim 1wherein said crosswise sensor guideway includes spaced parallel guiderails mounted on said guide frame parallel to said web guide roller,said carriage being slidably mounted on said spaced guide rails.
 6. In aweb sensing and guiding apparatus for laterally guiding a longitudinallytravelling web along a predetermined path of web travel including a webguide roller extending transverse to the travelling web to have the webwrap partway around the roller and exit therefrom in an exiting plane, amovable guide frame mounting the web guide roller on a stationarysupport structure for combined crosswise and angular movement relativeto said path of web travel, a web position sensor having a sensor headand operative to sense lateral deviation of a selected longitudinalcharacteristic of the web relative to said sensing head, and power meanscontrolled by said web position sensor for moving the guide frame andweb guide roller relative to said stationary support in a direction tocorrect deviation of said web from said desired path of web travel; theimprovement comprising: at least one crosswise guide bar mounted on saidmovable guide frame for movement therewith and extending crosswise ofsaid path of web travel and parallel to said guide roller, a sensorcarriage mounted on said crosswise guide bar for sliding movementtherealong, means mounting said web position sensor on said sensorcarriage with said sensing head arranged to sense the lateral positionof the web at a sensing location in said exiting plane of the web, alongitudinal sensor guide bar mounted on said stationary supportstructure and extending transverse to the plane of the exiting web, andsensor positioning means slidably engaging said longitudinal sensorguide bar and swivelly engaging said sensor carriage at a locationadjacent said sensing head for moving said sensor carriage along thecrosswise guide bar as the guide frame is moved relative to saidstationary support structure to maintain said sensing head at presetposition laterally of said path of web travel.
 7. A web sensing andguiding apparatus according to claim 6 wherein said web guide roller issupported for movement in a plane of motion transverse to the plane ofthe exiting web, said sensor positioning means defines a pivot axistransverse to said plane of motion and substantially aligned with theexiting plane of the web at said sensing location.
 8. A web sensing andguiding apparatus according to claim 7 wherein said web position sensorhas its sensing head positioned adjacent the periphery of said web guideroller to sense the lateral position of the web as it exits from theroller.
 9. A web sensing and guiding apparatus according to claim 6wherein said web position sensor has its sensing head positionedadjacent the periphery of said web guide roller to sense the lateralposition of the web as it exits from the roller.
 10. A web sensing andguiding apparatus according to claim 6 including a stationary guide barmounted on said stationary support structure and extending crosswise ofsaid path of travel, and means mounting said longitudinal guide bar foradjustment along said stationary guide bar.
 11. In a web positionsensing and guiding apparatus including an exiting web guide rollerextending transverse to a travelling web, a stationary supportstructure, means including a movable frame mounting said exiting webguide roller for movement in a plane of motion in which the exiting webguide roller is disposed generally tangent to a curved path about aneffective pivot center, said exiting web guide roller in themid-position thereof extending perpendicular to the desired path of webtravel and being arranged to have the travelling web extend partiallytherearound and exit therefrom in an exiting plane transverse to saidplane of motion, a web position sensor having a sensor head andoperative to sense lateral deviation of a selected longitudinalcharacteristic relative to the sensing head, and power means operated bythe web position sensor for moving said movable frame and said exitingweb guide roller in a direction to correct deviation of the web edgefrom said preselected lateral position, the improvement comprising: atleast one crosswise guide bar mounted on said movable frame for movementtherewith and extending parallel to said exiting web guide roller, asensor carriage mounted on said crosswise guide bar for sliding movementtherealong, means mounting said web position sensor on said sensorcarriage with said sensing head arranged to sense the lateral positionof the web at a sensing location in said exiting plane of the web, alongitudinal sensor guide bar mounted on said stationary support toextend transverse to the plane of the exiting web and parallel to theplane of motion of said exiting web guide roller, a follower slidablyengaging said longitudinal guide bar, and pivot means connecting saidfollower to said sensor carriage for relative pivotal movement about anaxis perpendicular to said plane of motion of the exiting web guideroller and substantially aligned with the plane of the exiting web atsaid sensing location for moving said sensor carriage along the sensorguide bar as the guide frame is moved relative to the stationary supportstructure to maintain the sensing head at a preset position laterally ofthe path of web travel.
 12. A web position sensing and guiding apparatusaccording to claim 11 including an entrance web guide roller mounted onsaid movable frame parallel to said exiting web guide roller andgenerally tangent to said effective pivot center.